Microphones: PDF

Microphones Notes 6 MICROPHONES 6.1 INTRODUCTION In the previous lessons you...

Microphones Notes 6 MICROPHONES 6.1 INTRODUCTION In the previous lessons you were acquainted with principles of sound, sound technology, digital vs analog etc. You learnt about the importance of acoustics. In this lesson, you will learn about the microphones, their classification etc.Also you shall learn about the major differences between dynamic and condenser microphones along with key considerations while selecting a particular type of microphone. 6.2 OBJECTIVES After studying this lesson you will be able to: Defines microphone and explain the functioning and working of microphones. Identify and explain the different types of microphones. Categorize the microphones based on their polar patterns. 6.3 CONCEPT The microphones (mic or mike in short) and the speakers are very common audio equipment. You see them not only in public meetings and conferences, you come across them even when you use your phone. The work of a microphone and a speaker are opposite of each other. A microphone converts sound vibrations into electrical entity (voltage/current) while a speaker converts the voltage/ current into sound vibrations by moving the diaphragm of the speaker and producing vibrations in the air. Basically a microphone has a diaphragm which moves when sound pressure pushes it. This movement can be converted into proportional voltage using several possible transducers. Here, a transducer is a device which receives electrical, mechanical or acoustic waves from one medium and converts them into related waves for a similar or different medium. Thus, it can be said that a microphone (mic) is a transducer that converts acoustical sound energy into electrical energy. Its basic function is therefore to convert sound energy into electrical audio signals which can be used for further processing. 76 Sound Technician  Microphones 6.4 CLASSIFICATION We can classify the microphones based on contruction/ directivity as shown below: Notes Microphones On the basis of Construction/type of Pick up or directionality transducer used properties (a) Condenser Microphone (a) Omni-directional (also capicitor/electrostatic microphone) (b) Unidirectional (b) Dynamic microphone (c) Bidirectional (c) Ribbon microphone (d) Carbon microphone (e) Piezoelectric microphone (f) Fiber optic microphone (g) Laser microphone (h) MEMS (Micro electrical mechanical sysstem 6.4.1 Microphones based on type of transducer /construction 1. Condenser Microphone called as Capacitor Microphone or Electrostatic Microphone also, is made up of two parallel very thin plates, positively and negatively charged respectively. The diagram 6.1 below shows a condenser microphone. + – Battery Sound waves Current Amplifier + – Backplate Fig: 6.1: Condenser microphone Sound Technician 77  Microphones It has a very thin diaphragm of thickness 1 to 10 micrometers. One micrometer (or micron) is one millionth of a meter or one thousandth of a millimeter. Close to Notes this plate (metallic or metalised plastic) stands another metallic plate with holes. These 2 plates act as electrodes and are kept at opposite polarities by supplying D C to behave as a condenser, they should be insulated from each other. When sound wave pushes the diaphragm, it vibrates and the capacitance of the condenser (or capacitor) changes. This is because the capacitance is proportional to the potential difference and inversely proportional to the separation between the plates. Any change in the separation changes the capacitance. The capacitance is also dependent upon the medium but as the medium here remains the same, so we ignore this parameter. The values of the resistance and the capacitance are chosen such that the change in voltage is immediately reflected in the voltage across the resistance in series. Any change in sound leads to change of the capacitance and leads to voltage change. The voltage is fed to an amplifier to amplify the level of the signal. Condenser microphones were invented in Bell Labs in 1916. 2. Dynamic Microphone: works on the principle of electro-mechanical induction. This type of microphone is called moving coil microphone also. Here a very small coil is used which is attached to a diaphragm and suspended in a magnetic field of a magnet as shown in the diagram: 6.2 below. When sound waves impinge on the diaphragm it vibrates and attached coil moves. This movement of the coil inside the magnetic field produces a emf across the terminals of the coil. The current so produced in the coil is in proportion to the sound. Magnet C O I L CURRENT SOUND WAVES DIAPHRRGM Fig. 6.2: Dynamic microphone 78 Sound Technician  Microphones 3. Ribbon Microphone: A ribbon microphone uses a corrugated ribbon made of a metal is suspended in a magnetic field as shown in the diagram: 6.3 below. Sound causes the ribbon to vibrate. This means change in magnetic flux through Notes the ribbon. This induces an electric current which drives a speaker. When this current is flown through a coil attaches to diaphragm of the microphone, the diaphragm vibrates and produces sound. Special materials developed using nano technologies are being used to make ribbons that will be light but strong. Being light improves the response to sound. The ribbon microphone senses pressure- gradient and not just pressure. Therefore, it detects sound from both sides. Magnet H S Pole Pieces Ribbon output S Connections Flexible Aluminum ribbon S Magnet Fig. 6.3: Ribbon microphone 4. Carbon microphone is also known as a carbon button microphone (or sometimes just a button microphone), use a capsule or button containing carbon granules pressed between two metal plates. 5. Piezoelectric microphone uses the phenomenon of piezoelectricity—the ability of some materials to produce a voltage when subjected to pressure—to convert vibrations into an electrical signal. 6. Fiber optic microphone converts acoustic waves into electrical signals by sensing changes in light intensity, instead of sensing changes in capacitance or magnetic fields as with conventional microphones 7. Laser microphones are often portrayed in movies as spy gadgets, because they can be used to pick up sound at a distance from the microphone equipment. 8. MEMS (Micro Electrical-Mechanical System) microphone is also called a microphone chip or silicon microphone. The pressure-sensitive diaphragm is etched directly into a silicon chip by MEMS techniques, and is usually accompanied with integrated preamplifier. Sound Technician 79  Microphones 6.4.2 Microphone Classification based on Polar Patterns Notes A microphone’s directionality or polar pattern indicates how sensitive it is to sounds arriving at different angles about its central axis. Some microphone designs combine several principles in creating the desired polar pattern. Generally the pick up pattern / Polar pattern of microphones fall in following three categories: (i) Omni-directional (ii) Uni-directional (iii) Bi-directional (i) Omni directional An Omni directional (or non-directional) microphone’s response is generally considered to be a perfect sphere in three dimensions as shown in the diagram: 6.4 below. In the real world, this is not the case. As with directional microphones, the polar pattern for an “omnidirectional” microphone is a function of frequency. Fig. 6.4: Omni directional (ii) Unidirectional A unidirectional microphone is sensitive to sounds from only one direction. The diagram 6.5 below shows a photo and Diagrams 6.7 to 6.11 illustrates a number of these patterns Fig. 6.5: University Sound US664A dynamic supercardioid microphone 80 Sound Technician  Microphones The most common unidirectional microphone is a cardioid microphone, so named because the sensitivity pattern is a cardioid. A cardioid microphone is effectively a superposition of an Omni directional and a Fig. 6.8 microphone; for sound waves Notes coming from the back, the negative signal from the Fig. 6.8 cancels the positive signal from the omnidirectional element, whereas for sound waves coming from the front, the two add to each other. Fig. 6.6: Bi-directional Fig. 6.7: Subcardioid Fig. 6.8: Cardioid Fig. 6.9: Hypercardioid Fig. 6.10: Supercardioid Fig. 6.11: Shotgun Sound Technician 81  Microphones A hyper-cardioid ( Fig 6.9) microphone is similar, but with a slightly larger figure-8 contribution leading to a tighter area of front sensitivity and a smaller lobe of rear Notes sensitivity. A super-cardioid microphone is similar to a hyper-cardioid, except there is more front pickup and less rear pickup. While any pattern between omni and Fig. 6.8 is possible by adjusting their mix, common definitions state that a hypercardioid is produced by combining them at a 3:1 ratio, while supercardioid is produced with a 5:3 ratio Shotgun Fig. 6.12: An Audio-Technica shotgun microphone Shotgun microphones (Fig 6.12 above) are the most highly directional. They have small lobes of sensitivity to the left, right, and rear but are significantly less sensitive to the side and rear than other directional microphones. This results from placing the element at the back end of a tube with slots cut along the side; wave cancellation eliminates much of the off-axis sound. Due to the narrowness of their sensitivity area, shotgun microphones are commonly used on television and film sets, in stadiums, and for field recording of wildlife etc. (iii) Bi-directional “Figure 6.8” or bi-directional microphones receive sound equally from both the front and back of the element. Most ribbon microphones are of this pattern. In principle they do not respond to sound pressure at all, only to the change in pressure between front and back; since sound arriving from the side reaches front and back equally there is no difference in pressure and therefore no sensitivity to sound from that direction. INTEXT QUESTIONS 6.1 1. A microphone (mic) is a transducer that converts acoustical …….energy into electrical energy (a) sound (b) noise ( c) mechanical. (d) Kinetic 2. Condenser microphone was invented at Bell Labs in …… (a) 1917 (b) 1916 (c) 1918 (d) 1919 82 Sound Technician  Microphones 3. A microphone’s directionality or polar pattern indicates how sensitive it is to (a) light (b) wind (c) sound (d) energy Notes 4. A unidirectional microphone is sensitive to sounds from ……..direction/directions (a) one (b) bi (c) multi (d) corner 5. ……….. microphones are the most highly directional (a) Ribbon (b) Carbon (c) Cardiod (d) Shotgun 6.5 FACTORS TO BE CONSIDERED WHILE SELECTING MICROPHONES Selecting on appropriate type of microphone is an important factor to control noise and produce quality sound. Factors which should be considered before selecting a microphone are: (i) Impedance: Generally, resistance is considered for circuits width dc source. In case ac supply is used and circuit contains inductor and capacitor in addition to resistors, impedance is considered. While selecting microphones impedance has involvement of frequency factor. The length of the cable used for microphone varies as per the impedance needed for a particular circuit or load. In most of the cases the wire impedance is kept low to increase the length of the cable for mic system. (ii) Frequency response: Frequency response is a microphone’s capability to service high and low frequency sounds, Ideally, quality microphone can receive frequencies ranging from 20 to 25,000 Hz. (iii) Pick up pattern: This refers to choice of sound from one or all directions. Different microphones are used to cater such varied requirement. (iv) Balanced and unbalanced microphone: Balanced microphones are used for professional recording purposes and they carry three wires while unbalanced connection use 2 wires i.e. central conductor carrying audio signal while shield/ basket carrying ground wire. (v) Sensitivity: it is the ability of microphone to pick up faint sounds and thus termed as sensitivity Highly sensitive microphones are used in studio while in field, low sensitivity based microphones are used as the chances of microphone being affected by strong wind are higher. Sound Technician 83  Microphones 6.6 WHAT HAVE YOU LEARNT Notes In this lesson you have learnt about microphones and fundamental principle of working of microphone. Different types of mics based up on their working principle were discussed to familiarize with the basic differences of microphones. Later pickup patterns of microphones were discussed based up on their applications. Different mics and their applications were also discussed. 6.7 TERMINAL QUESTIONS 1. What is a microphone? 2. What do you understand by ribbon microphone? 3. Explain a bi-directional microphone. 4. Differentiate between Omni and Uni-directional microphones 5. What are the precautions to be taken while handing microphones. 6.8 ANSWER TO IN-TEXT QUESTIONS 1. A 2. B 3. C 4. A 5. D 6.9 REFERENCES 1. Audio Production/Module4/Unit 13 Certificate in Community Radio Technology CEMCA 84 Sound Technician